PURPOSE: To attain accurate path check by adding a check code and a flag to represent the position of the check code, thereby preventing mis-production of a path checking alarm with a simple procedure.

CONSTITUTION: An addition means 102 adds the checking code and the flag to represent the position into a transmission signal and a detection means 103 detects the flag and the check code from the transmission signal. A comparing means 104 is triggered by the output of flag detection, compares the detection checking code with a set checking code, and it is discriminated that the path formation is normal when the codes are identical. Since the flag is shared for path checking code identification and for triggering the check code comparison means for path check in such a manner, the production and the detection of the path checking start signal are not required. Thus, the mis-production of a path checking alarm is prevented with a simple procedure to attain accurate path check.